The present application relates to sub-resolution alignment of images, for example, such as used in probing and editing integrated circuits.
An integrated circuit (“IC”) integrates a large number of electronic circuit elements on a single semiconductor substrate with high density: today's technology allows a minimum feature size on the order of 0.1 micron. During designing, prototyping, and testing an IC, circuit elements can be probed and edited. To probe or edit an IC using, for example, laser voltage probing (“LVP”) or focused ion beam (“FIB”), a circuit element first is located on the substrate of an IC under test. Typically, this step includes aligning corresponding features of two different images of the IC under test. The first image can be an acquired image that describes the actual position of the circuit. The second image can be derived from a computer-aided design (“CAD”) image that lays out the complicated map of circuit elements. In general, a CAD image is ideal representation of the IC and typically is generated by a human operator using a CAD system. Once the acquired image is aligned, or registered, with the CAD image, a conventional system can navigate, that is, steer, an IC probing device to a circuit element to be probed.
To acquire an image for alignment, an IC can be imaged, for example, by infrared (“IR”) light. Typically used for an IC with a flip-chip design, IR light can image the IC from the silicon side, i.e., through the substrate. To see through the substrate, which can be several hundred microns thick, silicon side imaging may use IR light with a wavelength of about one micron. Using an IR wavelength of about one micron, however, results in an acquired image of roughly the same resolution as the wavelength of the IR light used for imaging. That is, the resulting IR image has a resolution of about one micron. Such an IR image typically cannot adequately be used to resolve sub-resolution features, i.e., circuit elements that are smaller than the IR wavelength.
To locate sub-resolution features for IC probing or editing, an attempt can be made to align an IR image with the corresponding CAD image with sub-resolution accuracy. For example, a human operator can try to align an IR image with a CAD image visually. This method, however, typically gives an optimal accuracy of about one micron, which is essentially the same as the resolution of the IR image, and typically insufficient for LVP or FIB editing. For aligning IR and CAD images with sufficient accuracy, one can try standard alignment techniques, such as intensity correlation, edge detection or binary correlation algorithms. These techniques tend to give limited accuracy as well, because IR images may be distorted by light diffraction and other optical effects. Alignment of an IR and a CAD image may be further complicated by substantial intensity variations. Intensity on the IR image can depend on several parameters, including thickness and reflectivity of different layers. Furthermore, IR images may have optical ghosts that may cause an alignment method to produce incorrect results.